- 2 TUBULAR BELT CONVEYOR TECHNICAL FIELD The present invention relates to a tubular belt 5 conveyor into which an endless circulating belt is rolled up for conveying materials. BACKGROUND OF THE INVENTION JP9-169423A discloses that magnetic substances are 10 provided in each side of a trough-shaped running conveyor belt around which a fixed body includes a plurality of magnetic sensors circumferentially so that output differences from the magnetic sensors enable how the belt meanders to be detected. 15 However, a number of magnetic sensors are required, so that the support structure therefore becomes more complicated and increases cost. 20 A tubular belt conveyor into which a flat belt is rolled up dents inward by its own weight and stress, but the above publication does not disclose how to detect a depression. 25 SUMMARY OF THE INVENTION In a first aspect, the invention provides a tubular belt conveyor comprising: a tubular belt formed by rolling up a flat belt, the tubular belt having an overlapped portion comprising a 30 first side edge and a second side edge which the first side edge overlaps, and a permanent magnet disposed in either the first side edge or the second side edge, the tubular belt being adapted to allow materials to be 2474615_1 (GHMatters) 29/11/10 - 3 conveyed in the tubular belt; a plurality of magnetic sensors arranged circumferentially into two zones around the tubular belt, the two zones comprising an allowable zone and an 5 unallowable zone, each magnetic sensor being adapted to measure a magnetic strength of the permanent magnet; detecting means connected to each of said plurality of magnetic sensors, the detecting means being adapted to detect whether the tubular belt has twisted beyond 10 permissible limits based on the magnetic strengths measured by the magnetic sensors in the unallowable zone, and to detect whether the overlapped portion has deviated inward based on an absolute value of the magnetic strengths measured by the magnetic sensors; and 15 a display connected to the detecting means to display the magnetic strength of the permanent magnet measured by each of said plurality of magnetic sensors, the display being adapted to display a peak on a line corresponding to a magnetic sensor arranged in the 20 unallowable zone if the tubular belt has twisted beyond permissible limits. In an embodiment, the tubular belt conveyor further comprises a warning device connected to the twist 25 detecting means to warn when the twist exceeds the permissible limits. In an embodiment, the tubular belt conveyor further comprises an operation stopping device connected to the 30 twist detecting means to stop the tubular belt conveyor if the twist exceeds the permissible limits. In an embodiment, an intermediate zone is provided 2474615_1 (GHMatters) 29/11/10 4 between the allowable zone and the unallowable zone, the peak appearing on a value detected by the magnetic sensor in the intermediate zone to show that the twist of the conveyor belt becomes cautious. 5 In an embodiment, a magnetic sensor is provided in an intermediate zone between the allowable zone and the unallowable zone, the peak appearing in the display on a line corresponding to the magnetic sensor in the 10 intermediate zone to show that the twist of the conveyor belt is cautious. In an embodiment, an overlapped portion of the magnetic sensors is provided between the allowable zone 15 and the unallowable zone thereby showing that the twist of the conveyor belt is cautious. In an embodiment, each of said plurality of magnetic sensors comprises a loop coil. 20 In an embodiment, the warning device includes a plurality of lamps emitting different colours depending on how to twist the conveyor belt. 25 In a second aspect, the invention provides a conveying method comprising: forming a tubular belt by rolling up a flat belt having first and second side edges such that said first and second side edges form an overlapped portion of the 30 tubular belt, a permanent magnet being disposed in either the first side edge or the second side edge, the tubular belt being adapted to allow materials to be conveyed in the tubular belt; detecting with detecting means connected to each of 35 a plurality of magnetic sensors whether the tubular belt has twisted beyond permissible limits based on the magnetic strengths measured by the magnetic sensors in the 2558693_1 (GHMatters) 4/02/11 5 unallowable zone, said magnetic sensors being arranged circumferentially into two zones around the tubular belt, the two zones comprising an allowable zone and an unallowable zone, each magnetic sensor being adapted to 5 measure a magnetic strength of the permanent magnet; detecting with said detecting means whether the overlapped portion has deviated inward based on an absolute value of the magnetic strengths measured by the magnetic sensors; and 10 displaying a peak on a line corresponding to a magnetic sensor arranged in the unallowable zone when the tubular belt has twisted beyond permissible limits, with a display connected to the detecting means and adapted to display the magnetic strength of the permanent magnet 15 measured by said plurality of magnetic sensors. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly ascertained, embodiments will now be described, by way of 20 example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of the first embodiment of the present invention. 25 Fig. 2 is an enlarged vertical sectional view and a block diagram of detecting means of the first embodiment of a tubular belt conveyor according to the present invention. 30 Fig. 3 is an enlarged vertical sectional view and a block diagram of detecting means in the second embodiment of the present invention. 35 Fig. 4 is an enlarged vertical sectional view and a block diagram of detecting means in the third embodiment of the present invention. 2558693_1 (GHMatters) 4/02/11 - 6 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiments of the invention will be described with respect to accompanying drawings. 5 Figs. 1 and 2 show the first embodiment of the present invention. In Fig. 1, an endless conveyor belt 3 of a tubular belt conveyor is wound around a front pulley 1 and a rear 10 pulley 2, and either of the front pulley 1 and the rear pulley 2 is rotated by drive means (not shown) to enable the conveyor belt 3 to circulate. An upper carrying part and a lower returning part of the conveyor belt 3 pass through a plurality of guide frames (not shown) in which a 15 plurality of guide rollers are arranged like a ring or trough. So the conveyor belt 3 is rolled up into a tubular shape while one side edge 3a lies on the other side edge 3b, and materials 5 supplied from a hopper 4 above the rear pulley 2 is wrapped and transferred in the 20 tubular belt. A plurality of permanent magnets 6,7 are embedded in the conveyor belt 3 at proper intervals longitudinally of the conveyor belt 3 with opposing different magnetic poles 25 in one side edge 3a and the other side edge 3b when the belt is rolled up in the tubular shape. In the first embodiment, in Fig. 2, the upper and lower parts of all the permanent magnets 6 in the side 30 edges 3a,3b of the tubular conveyor belt 3 are N- and S poles respectively. Nr\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 7 The magnetic poles may be reversed or may be different one by one longitudinally. Either of the permanent magnets 6,7 may be made of steel. 5 In Figs. 1 and 2, the permanent magnet 6 lies on the permanent magnet 7 when the conveyor belt 3 is rolled up, but may be disposed at positions different from each other or staggered. 10 In Fig. 2, on a fixed body(not shown) provided at an area through which the tubular belt 3 passes, a plurality of sensors 8 for detecting magnetic strength by the passing permanent magnets 6,7 are provided circumferentially around the conveyor belt. 15 In the embodiment, the magnetic sensors 8 comprise eight loop coils 8-1 to 8-8. In the outer circumferential area around the tubular belt 3, there are an allowable zone A in which a twisted overlapped portion 3c in which 20 the side edge 3a lies on the side edge 3b can run; and an unallowable zone B. There are three loop coils 8-4,8-5,8 6 in the allowable zone A, and five loop coils 8-1,8-2,8 3,8-7,8-8 in the unallowable zone B. 25 Each of the loop coils 8-1 to 8-8 or magnetic sensor 8 is connected to detecting means 9 for finding a twist and a depression, and magnetic strength detected by the loop coils 8-1 to 8-8 is shown in a display 10 connected to the detecting means 9. 30 The detecting means 9 comprises twist detecting means 11 and depression detecting means 12. The twist detecting means 11 compares magnetic strength detected by the N:\Melbourne\CaseB\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 8 magnetic sensors 8 to identify a twist of the conveyor belt 3 depending on the magnetic sensor 8 which detects the peak. 5 In an untwisted state shown by solid lines in Fig. 2, the overlapped portion 3c of the conveyor belt 3 is positioned at the top, and the permanent magnets 6,7 runs close to the loop coil 8-5. In the display 10, the peak P1 is generated on the fifth line corresponding to the 10 loop coil 8-5. The other loop coils 8-1 to 8-4 and 8-6 to 8-8 are away from the permanent magnets 6,7, and the first to fourth lines and sixth to eighth lines in the display 10 corresponding thereto are substantially straight. 15 By detecting with the twist detecting means 11 that the peak P1 is generated in a line detected by the loop coil 8-5, it is found that the overlapped portion 3c of the conveyor belt 3 is positioned in the range facing the loop coil 8-5, which means that the conveyor belt 3 is 20 nearly untwisted. If the conveyor belt 3 is twisted, for example, as shown by a dotted line, the overlapped portion 3c of the conveyor belt 3 is deviated left by 90 degrees from the 25 solid line, the peak P2 is created in a line detected by the loop coil 8-3 facing the overlapped portion 3c, as shown in the display 10 in Fig. 2. By the detection of the twist detecting means 11, it is found that the overlapped portion 3c of the conveyor belt 3 is positioned 30 in the range facing the loop coil 8-3, which means that the twist of the conveyor belt 3 exceeds permissible limits. N:\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 9 The conveyor belt 3 is twisted, so that the overlapped portion 3c is positioned in front of the loop coils 8-4 or 8-6. So the peak is created in the loop coil 8-4 or 8-6, so that it is found that the conveyor belt 3 5 is twisted within the permissible limits. If the twist detecting means 11 detects that the twist of the conveyor belt 3 goes beyond the permissible limits, a warning device 13 connected to the detecting 10 means 9 or an operation stopping device 14 for stopping the whole tubular conveyor will work. The depression detecting means 12 identifies that the overlapped portion 3c of the conveyor belt 3 moves inward 15 or a depression is created in a circular shape depending on an absolute value of the peak P1 detected by the magnetic sensor 8. In the embodiment, the absolute value of the peak P1 20 of magnetic strength detected by the magnetic sensor 8 or an amplitude of the peak P1 in the display 10 in Fig. 2 goes down below a predetermined limit D, so that it is found that the overlapped portion 3c of conveyor belt 3 is bent inward. 25 The depression detecting means 12 detects a depression in the conveyor belt 3 to allow the warning device 13 and the operation stopping device 14 to be actuated to stop the whole tubular conveyor in such a 30 manner that a twist of the conveyor belt 3 is detected by the twist detecting means 11. N:\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 10 According to the first embodiment of the present invention, by the permanent magnets 6,7 at the side edges 3a,3b of the conveyor belt 3, a twist of the tubular conveyor belt 3 can be detected readily and efficiently. 5 Only the permanent magnets 6,7 may be used for detection. Without a special sensor, it can be found that the overlapped portion 3c of the tubular conveyor belt 3 deviates inward which means a depression in the conveyor belt 3. 10 Furthermore, in the first embodiment, when the conveyor belt 3 is in a tubular shape, the permanent magnets 6,7 at the side edges 3a,3b of the conveyor belt 3 are attracted to each other to prevent the side edges 15 3a,3b of the conveyor belt 3 from moving away from each other, so that the conveyed materials 5 is prevented from dispersing. The outer circumferential area of the conveyor belt 3 20 is divided into the allowable zone A and unallowable zone B in which the magnetic sensors 8 are provided, so that it can easily be detected that a twist of the conveyor belt 3 exceeds the permissible limits. Thus, the warning device 13 and stopping device 14 can prevent malfunction. 25 Fig. 3 shows the second embodiment of the present invention. The same numerals are allotted to the same members as those in the first embodiment, and detailed description thereon will be omitted. The same will apply 30 to the third embodiment, which will be described later. In this embodiment, intermediate zones C,C are provided between an allowable zone A and an unallowable N:\Melbourne\Case\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 11 zone B. Magnetic sensors 8 such as loop coils 8-1,8-2,8 3,8-4 are provided in order of the unallowable zone B, intermediate zone C, allowable zone A and intermediate zone C. If the loop coil 8-2 or 8-4 of the intermediate 5 zones C detects the peak of magnetic strength, twist detecting means 11 will identify a twist of the conveyor belt 3 under a caution. In the embodiment, a warning device 13 comprises a 10 blue lamp 13a, a yellow lamp 13b and a red lamp 13c. The blue lamp 13a will turn on if a twist of the conveyor belt 3 is identified to be within an allowable zone, the yellow lamp 13b will turn on and off if a twist of the conveyor belt 3 is identified to be cautious, and the red lamp 13c 15 will turn on and off if a twist of the conveyor belt 3 is identified to be dangerous beyond the permissible limits. A twist of the conveyor belt 3 can be detected more precisely and in more detail than what comprises only 20 allowable zone and allowable zones. Fig. 4 shows the fourth embodiment of the present invention. 25 The outer circumferential area around a tubular conveyor belt 3 is divided into two zones of an allowable zone A and an unallowable zone B, each of which comprises a magnetic sensor 8 such as loop coils 8-1 or 8-2 respectively. Between the zones A and B, one 30 circumferential edge of the magnetic sensors 8 lies on the other circumferential edge to form an overlapped portions 20. If permanent magnets 6,7 faces the overlapped portion 20 of the magnetic sensor 8, the magnetic sensors 8,8 will N3\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 12 detect the substantially equal-magnitude peaks P1,P2 as shown in a display 10 of Fig. 4 to allow twist detecting means 11 to identify a cautious position where an overlapped portion 3c of the conveyor belt 3 is placed in 5 a boarder of the zones A,B. By the only two magnetic sensors 8, the twist of the conveyor belt 3 can be detected exactly in detail similar to the second embodiment comprising the four magnetic 10 sensors 8. In the second embodiment, between the adjacent zones A-B, B-C, C-D, D-A, an overlapped portion of the magnetic sensors 8 face the permanent magnets 6,7 of the conveyor 15 belt 3. The adjacent magnetic sensors 8,8 detects substantially the same magnitude peaks to identify that the overlapped portion 3c of the conveyor belt 3 is placed at a boarder of the adjacent zones to enable a twist of the conveyor belt 3 to be detected exactly in more detail. 20 The foregoing relates to three embodiments of the present invention and is only for illustration only. Various changes and modifications may be made without departing from the scope of claims. 25 For example, either of the twist detecting means 11 and depression detecting means 12 may be omitted. The magnetic sensor 8 may be a hall element, a 30 gaussmeter or an MI sensor instead of the loop coil. The permanent magnet may be a sintered magnet or a magnet sheet. N:\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08 - 13 The sintered magnet is suitable for more precise detection, and a rubber magnet sheet is suitable for adherence and durability. 5 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as 10 "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 15 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. N.\Melbourne\Cases\Patent\77000-77999\P77252.AU\GH FIRST SPECIFICATION.doc 2/04/08